I think the game would be better if it were more like the games. If you had the option of battling trainers, weakening wild Pokemon to make them easier to catch, and adding trading.
Battling trainers and trading, sure, but I have to really disagree with weakening wild Pokémon. D: Remember, a lot of the game is meant to be done on the move, and wild Pokémon battles in the games take a bit of time. If that feature was added, then you'd really have no choice but to stop for minutes at a time and concentrate on your screen instead of the world around you as you select the right moves to weaken a Pokémon, keep an eye on a its HP bar,
and toss balls at it. In other words, capturing Pokémon would be impossible to do if you were walking or in a car or so forth unless you really didn't mind focusing on only one for minutes at a time. You'd
need to stop to do it.
Granted, yes, with the capture rates as they are, it takes a bit of time to catch Pokémon, but I'm saying it'll take
longer if battling was added to the mix. And yeah, I know folks already battle at gyms, but gyms are typically landmarks. You're
supposed to stop at them. Not so with Pokémon, which pop up randomly as you go.
That said, to answer your question, I'd love to see the candy system reworked a little. As it stands, it takes quite a bit of candies to evolve and power up Pokémon, but this means it's next to impossible to do it for rarer stuff. For example, you can totally have an army of Raticate and Pidgeotto, but God help you if you want to have more than one Charizard and you live in the boonies. Maybe it'll get better once trading is implemented (because, knowing the GO community, there probably
will be people who would be willing to shuttle rare Pokémon out to folks who need them), but ngl, it'd be so much easier if evolving and powering up didn't hinge on you getting a ton of candies of an extremely specific type. In other words, I'd love to see either a reduction in candy requirements
or a generalization of candies—meaning I'd love for the system to need less candies, or I'd love a system where you can collect and feed, say, Grass Candies to Oddish, Bellsprout, and Bulbasaur, rather than Oddish Candies to Oddish, Bellsprout Candies to Bellsprout, and Bulbasaur Candies to Bulbasaur. Sure, in some cases, that would mean you could feed either of
two different types of candies to a Pokémon (for example, either Grass
or Poison to Bulbasaur), and in others, this might be a little messy for an evolution line (when it's eventually added, Surskit will take Bug and Water Candies, but its evolution will need Bug and Flying), but it'd still be easier than running around, trying to collect 150 Bulbasaur Candies.
Also, related to another suggestion...!
Revives, give a point to them, I've never even used one.
They're for gym battles, tbqh. XD But I
do agree that there should be
less of them. You only really need them if you let your Pokémon faint while trying to take down a gym (which is kinda sorta a huge part of the game), but if you play your cards right, so to speak, you probably won't need 108937302 of them, contrary to what Pokéstops seem to think. Those item slots can totally be taken up by more balls or even potions (which you'll tend to use more of than revives).